1. Field of the Invention
The present invention relates to carrier frequency and phase, acquisition and tracking circuits for use in phase locked loop systems and more specifically to such circuits for use in phase locked loop systems with low carrier signal to noise ratios.
2. Description of the Prior Art
Carrier signal acquisition and tracking systems are known in the art. In particular, techniques for the acquisition and tracking of noisy signals such as those received from space or satellite vehicles are well established, see, for example, U.S. Pat. Nos. 4,457,006 Global Positioning System Receiver and 4,453,165 Differential Doppler Receiver, both issued to Reuben E. Maine and assigned to the assignee of the instant invention. In such systems a received RF carrier is typically modulated by a digital code sequence whose bit rate is much higher than the information signal bandwidth. One of the preferred modulation formats for these carriers is the bi-phase modulation technique in which a binary 1 and a binary 0 are represented by alternate 180.degree. phase shifts. The signal bandwidth of such systems must allow for initial frequency uncertainty due to doppler shifts and non-coherent frequencies between the transmitting and receiving systems. Such bandwidths permit noise as well as information into the receiving system. In order to recover the information with a high probability of detection, the receiving system local signals must be made coherent with the received or incoming signals. This is generally accomplished with carrier frequency and phase, acquisition and tracking techniques.
In general, noisy signals are acquired and tracked with techniques which employ coherent processes, wherein a local reference is compared with the information signal to generate corrective signals for altering the frequency and phase of the local reference to match the frequency and phase of the information signal. Prior art techniques for providing coherent signal reception are effective but require relatively complex circuitry.
One of the better known and documented signal comparator and error detector circuits for coherent reception is the Costas loop, as described in the book "Digital Communications by satellite" by James J. Spilker, Jr. and published by Prentice-Hall in 1977. Costas loop circuitry, however, is relatively complex and, at least in part, utilizes analog techniques. A pair of signal multipliers, operating in phase quadrature, square the input signal. These multipliers are followed by circuitry which filter unwanted product signals and generate error correcting signals that are utilized to provide coherent reception. In some applications the error signals are filtered and amplified in analog circuitry to directly control the local signal, while in other applications the error signals are converted to digital format by an analog-to-digital converter for microprocessor control of the local signal.
In contrast, the circuit of the present invention is completely digital, requires no multiplication or signal squaring, and requires only counters and attendant digital logic functions to filter and process error correcting signals.